Bipolar transistor and method for producing same

ABSTRACT

The aim of the invention is to provide for a bipolar transistor and a method for producing the same. Said bipolar transistor should have minimal base-emitter capacities and very good high frequency characteristics. The static characteristics, especially the base current ideality and the low frequency noise, of a bipolar transistor with weakly doped cap layer ( 116 ) should not significantly deteriorate and process complexity should not increase. According to the invention, the problem is solved by inserting a special doping profile in a cap layer ( 116 ) (cap doping) which has been produced epitaxially. A minimal base emitter capacity and very good high frequency characteristics can be obtained by means of said doping profile. At the same time, the efficiency of the generation/recombination active boundary surface between the cap layer ( 116 ) and the isolator ( 117 ) in the polysilicon overlapping area in the relevant working area of the transistor is reduced and the base current ideality is improved. The section at the base side in the cap layer ( 116 ) has a preferred thickness of between 20 nm and 70 nm and is only doped weakly, preferably less than 5·10 16  cm −3 . Said section is crucial for the good high frequency characteristics.

The invention relates to a bi-polar transistor and a procedure, for itsmanufacture.

BACKGROUND OF THE ART

The implementation of epitaxially manufactured silicon-germaniumhetero-bi-polar transistors (SiGe HBT) and the resulting cost-reducingsimplification of the technological processes have lately provided a newimpetus for a further development of Si bi-polar transistors. In thisrespect, the combination of an epitaxially produced base with theprocess-simplifying possibilities of the single polysilicon technologyoffers an attractive direction of development.

In comparison with conventional base profiles produced by implantationor diffusion, silicon-germanium base layers made by epitaxy allowproducing, simultaneously, smaller base widths and base layer resistancewithout unusable small current gains or high leakage currents. Thetechnology allows implementation of a concentration of the active dopingagent of up to 1×10²⁰ cm⁻³, as is described—for example—in A. Schüppen,A. Gruhle, U. Erben, H. Kibbel und U. König: 90 GHz fmax SiGe-HBTs, DRC94, page IIA-2, 1994. However, in order to prevent leakage currents dueto tunnel processes, a low-doped region is required between thehigh-concentration zones of the emitter and the base. As a matter offact, if the base doping exceeds the value of 5×10¹⁸ cm⁻³, and if thehigh concentration of the emitter reaches down to the base—as is usualwith implanted base profiles—the consequence is the existence ofunacceptably high tunnel currents. As opposed to implanted baseprofiles, the application of epitaxy allows, simultaneously and withoutany problems, the production of narrow base profiles and a low-dopedregion (cap layer).

FIG. 1 illustrates the emitter zone of a SiGe HBT. This transistordesign reflects typical characteristics of a single poly-siliconprocess. An SiGe base 12 and subsequently a cap layer 13 were depositedover a monocrystal collector zone 11. FIG. 1 does not show a lateralinsulation of the transistor zone. If semiconductor material grows bothon the monocrystal substrate 11 and on the insulator zone—not shown inthe picture—(i.e., differential epitaxy), it is possible to utilize thegrown semiconductor layers as a connection between a contact on theinsulation zone and the inner transistor. Such a connection should be,designed with as low impedance as possible. This is why it would beadvantageous if the epitaxial layer thickness could be set upindependently from the base width. A poly-silicon or an α-silicon layer15 is deposited on an insulation layer 14, in which emitter windows wereetched by means of a wet-chemical etching process. During the depositionor subsequently, the α-silicon layer 15 obtains—by implantation—a dopingof the emitter's conductivity type and serves as diffusion source forthe emitter doping 16 in the monocrystal substrate. Insulator layer 14is applied in order to prevent damage to cap layer 13 during thestructuring of the polycrystal α-silicon layer 15 performed later. Inthe overlapping region 17—a zone between the edge of the emitter windowand the outer delimitation of the structured poly-silicon or α-siliconlayer 15, a layer, sequence arises consisting of semiconductor material,insulator material and semiconductor material. Depending on the dopingof the cap layer 13, the interfacial charges and the recombinationproperties of the surface as well as on the operation conditions of thetransistor, this design can cause—analogous to a MOS capacity—anenhancement but also a depletion of mobile charge carriers on thesurface of the cap layer 13. With a forward-current base-emitter diode,this can affect both the ideal nature of the base current and thelow-frequency noise properties. Under certain circumstances, generationcurrents and breakdown voltage in the non-conducting direction can beaffected. The condition that—due to the tunnel (currents) danger—thedoping agent concentration should not exceed the value of 5×10¹⁸ cm⁻³leads to the question, by means of which procedure this zone should besuitably doped. The following text discusses the variants for SiGe HBTso far known: homogeneous n-doping or p-doping near the tunnel limit orquasi undoped zones (i-zones). A. Chantre, M. Marty, J. L. Regolini, M.Mouis, J. de Pontcharra, D. Dutrtre, C. Morin, D. Gloria, S. Jouan, R.Pantel, M. Laurens and A. Monroy: A high performance low complexity SiGeHBT for BiCMOS integration, BCTM '98, 1998, pages 93-96 uses a p-dopingof about 5×10¹⁸ cm⁻³. This results in a decisive disadvantage in thatthe thickness of, the cap layer must be set up within a tolerance rangeof a few nanometers from the penetration depth of the doping agentdiffusing from the poly-silicon emitter layer. Greater cap layerthickness values (which would be advantageous for a low-impedanceconnection between the inner base and a connector in the insulationzone) are not possible since it would negatively affect the effect ofthe germanium profile. A. Gruhle, C. Mähner: Low l/f noise SiGe HBTswith application to low phase noise microwave oscillators, ElectronicsLetters, Vol. 33, No. 24, 1997, pages 2050-2052 uses a cap layer 100 nmthick with an n-concentration of 1-2×10¹⁸ cm⁻³. EP-A-0 795 899 indicatessimilar conditions, where preferably a cap layer of a thickness of 70 nmwith a n-doping concentration of 2×10¹⁸ cm⁻³ is used. Although thisvariant eliminates the problem of the thickness tolerance, and avoidsthe danger of tunnel currents by reducing the doping agent concentrationin the cap layer, it still does not take full advantage of thepossibilities of reducing the base-emitter capacity.

This disadvantage can be eliminated by not doping the cap layer as isdescribed, for example, in B. Heinemann, F. Herzel and U. Zillmann:Influence of low doped emitter and collector regions on high-frequencyperformance SiGe-base HBTs, Solid-St. Electron, 1995, Volume 38(6),pages 1183-1189. However, it can easily lead to a depletion of theaforementioned overlapping region 17. These connections are explained infurther text by means of a two-dimension design element simulation.

FIG. 2 shows the simplified transistor design used in the simulation.The electrical effect of the oxide semiconductor surface in theoverlapping region is modeled by means of a positive surface chargedensity of 1×10¹¹ cm⁻² and a surface recombination speed of 1000 cm/s.FIG. 3 illustrates vertical profiles along a section line horizontal tothe overlapping region. The profiles show three doping variants in thecap layer 13 and a p-doped SiGe base 12 identical for all three cases.The following cap doping cases are compared: a quasi undoped cap layer13 (profile i) and two homogeneous n-dopings (profile n1 with 1×10¹⁸cm⁻³ and profile n2 with 2×10¹⁷ cm⁻³). FIG. 4 shows the transitionfrequency as a function of the collector current for various dopingvariants. Especially with small collector currents, an increase intransition frequency with a falling doping level in the cap layer 13 canbe noticed. While profile i provides relatively best transitionfrequencies, it has, however, the disadvantage that the ideal nature ofthe base current (FIG. 5) is noticeably affected in comparison with theother profiles.

SUMMARY OF THE INVENTION

The task of this invention is to indicate a bi-polar transistor and aprocedure for its manufacture that eliminates the describeddisadvantages of conventional arrangements, in order to achieveespecially minimal base-emitter capacities and best high-frequencyproperties without noticeably affecting the static properties of thebi-polar transistor with a low-doped cap layer—above all the idealnature of the base current and low-frequency noise—and withoutincreasing the process complexity.

This invention fulfills this task by introducing a special dopingprofile into an epitaxially produced cap layer (cap doping). This dopingprofile allows achieving a minimal base-emitter capacity and besthigh-frequency properties, but also restricts the effect of ageneration-active and recombination-active interface between the caplayer and the insulator in the overlapping poly-silicon region in theinteresting function range of the transistor, and improves the idealnature of the base current.

Decisive for the good high-frequency properties is the base-side sectionin the cap layer of a preferable thickness between 20 nm and 70 nm withlow-concentration doping, preferably less then 5×10¹⁶ cm⁻³.

On the emitter side, the cap layer is doped more highly. If the dopingagent is of a conductivity type like the base layer, the doping agentconcentration applied in the cap layer is preferably less than 5×10¹⁸cm⁻³ in order to prevent tunnel currents.

The cap doping profile is preferably introduced by implantation in situduring the epitaxy procedure.

The characteristics of this invention are clear from the claims and alsofrom the description and the drawings, where each characteristic—eitherindividually or several characteristics in the form ofsub-combinations—represent patentable designs, for which protection isdemanded herein. Design examples are illustrated in the drawings and areexplained in more detail in further text.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show:

FIG. 1: A schematic illustration of the emitter zone of a bi-polartransistor manufactured with a single poly-silicon technology with anepitaxially deposited base,

FIG. 2: A schematic illustration of the simulation region for thebi-polar transistor according to FIG. 1 (not in correct scale),

FIG. 3: Vertical doping profiles under the overlapping region forvarious cap doping levels,

FIG. 4: Transition frequency as a function of the collector currentdensity for various doping profiles,

FIG. 5: Graphs for various doping profiles,

FIG. 6: Vertical doping profiles under the overlapping region forvarious cap doping levels,

FIG. 7: Graphs for various doping profiles,

FIG. 8: Transition frequency as a function of the collector currentdensity for various doping profiles, and

FIG. 9: A schematic illustration of a bi-polar transistor during themanufacturing process.

DETAILED DESCRIPTION OF THE INVENTION

The characteristics and effects of the cap doping profiles according tothis invention are described by means of a two-dimensional elementsimulation on an npn SiGe HBT. The explanation can be applied to a pnptransistor accordingly.

FIG. 6 shows characteristic examples for the vertical profiles (asproposed herein) in the cap layer 13 along a section line horizontal tothe overlapping region. The doping agent concentration of the cap“profile p1” is growing in direction to the surface of the cap layer andreaches there its maximum concentration with about 9×10¹⁷ cm⁻³, whereasthe box-like profiles “p2” and “n3” are 10 nm wide and doped with 2×10¹⁸cm⁻³. The profiles p1 p2 are of a p conductivity type, profile n3 is ofn-type. FIG. 7 shows “Gummel” graphs to profiles p1, p2 and n3, when thecharacteristics of profile 1 from FIG. 5 were taken; over forcomparison. FIG. 7 shows a clear improvement in the characteristics ofthe base current when cap doping is used as compared with the behaviorof profile i. Dynamic calculations to these profiles lead to the resultsshown in FIG. 8: Unlike the homogeneous dopings n1 and n2 withconcentrations of 1×10¹⁸ cm⁻³ and 2×10¹⁷ cm⁻³, profiles p1, p2 and n3demonstrate no noticeable deterioration of transition frequencies incomparison with profile i. Decisive for the good high-frequencyproperties is the section in the cap layer of a preferable thickness ofat least 20 nm with low-concentration doping, preferably less then5×10¹⁶ cm⁻³. The results indicate that, in the example shown here,n-profiles and n-profiles in: the cap layer can achieve comparableresults.

In practice, the decision which doping type should be applied depends onthe circumstance, e.g., of which type and density are the charges on theSi/insulator interface or in the insulator, or which manufacturingprocedure can be used for the cap doping process. So, e.g., the proposedprofiles can be introduced by implantation. However, this variant shouldbe preferred only if the effects of point defects on the base profilecan be controlled. Should the curing of the point defects lead to anincreased diffusion of the base doping from the SiGe layer and,therefore, to an unacceptable deterioration of the electricalproperties, other doping variants are required. For example, an in situdoping during the epitaxy process is possible. During this procedure,the type of the cap doping is co-determined by the safety and simplicityof the deposition process. The following text explains the manufacturingof a bi-polar transistor according to this invention on the example ofan npn SiGe HBT. The revealed procedure can, be applied to pnptransistors as well. In addition, according to this invention it is alsopossible not to use an epitaxy process on the base layer and, instead,introduce the base profile by implantation before the epitaxialmanufacturing of the cap layer.

As illustrated in FIG. 9, structured regions consisting of a collectorregion 112 of the conductivity type II and an insulation region 113(which surrounds the collector region 112) were produced on amonocrystal substrate layer 111 of the conductivity type I. If theemitter and the collector are, e.g., n-conductive, the base is of thep-type and vice versa. Various suitable insulation techniques are knownsuch as LOCOS processes, spaced mesa arrangements or deep or flat trenchinsulation.

On the basis of a differential epitaxy process, a buffer layer 114, aSiGe layer with in-situ doping of the base layer 115 of the conductivitytype I and a cap layer 116 are is applied on the entire surface.

While the buffer layer 114, the base layer 115 and the cap layer 116grow—as monocrystal materials—on the silicon substrate, polycrystallayers 114/1, 115/1 and 116/1 arise over the insulation zone 113. Afterphotolithographic masking, dry-etching techniques are applied to removethe epitaxy layer in those regions in which no transistors arise.

If a selective epitaxy process is used instead of differential epitaxy,where growth occurs exclusively on the silicon underground, thestructuring of the epitaxy layer stack is eliminated.

In the following step, the silicon regions with an insulation layer 117are exposed. This can be achieved by means of thermal oxidation and/ordeposition. Layer stacks of dielectrics such as silicon oxide andsilicon nitride can be applied. Besides that, the electricallyconductive layer can be covered with a poly-silicon layer in order tomaintain additional flexibility for the process at a later stage.

Essential from the point of view of the procedure according to thisinvention is the implementation of the cap doping profile in anepitaxially produced cap layer. There is a possibility to introducesimilar profiles, as shown in FIG. 6, in situ during the epitaxyprocess. Furthermore, a flat profile can be produced by implantationbefore or after the production of the insulation layer 117. In addition,various procedures for the diffusion of such profiles are also known.This can also be performed by means of an insulation layer highlyenriched with the doping agent. A diffusion step can occur before orafter further procedure steps. The use of diffusion-preventingingredients in the collector, the base and the cap layer 116 such ascarbon is especially useful if certain processes are used such asimplantation, diffusion or thermal oxidation, which can cause anaccelerated diffusion of the doping agents.

The transistor manufacturing process can now proceed with thestructuring of a coating mask for the opening of the emitter window. Inthis step, the cover layers are removed in well-known etchingprocedures. In order to achieve good transistor properties, preferablywet-etching techniques should be used to expose the semiconductorsurface.

The process continues with the deposition of an amorphous silicon layerfor the creation of a poly-silicon emitter. This layer can be doped insitu by implantation during or immediately after the deposition.

The process then continues with conventional steps of structuring,implantation and passivation. The required high-temperature steps aretaken to cure implantation defects and to form the poly-emitter. Themanufacturing process is completed with the opening of the contactapertures for the emitter, the base and the collector and with astandard metallization of the transistor contacts.

This invention explains, on the basis of concrete design examples, abi-polar transistor and a procedure for its manufacture. However, noticemust be taken that this invention is not restricted to the particularsof the description of any particular design example, since, within thepatent claims, changes and deviations are also subject to patentprotection.

What is claimed is:
 1. A procedure for the manufacture or a bi-polartransistor, said procedure comprising the steps of: producing structuredregions consisting of a collector region and an insulation region thatsurrounds the collector region, on a monocrystal substrate layer;producing a base layer and, by means of epitaxy, a cap layer over thecollector region; depositing an insulation layer is deposited over thecap layer, the insulation layer being opened in an area of an effectiveemitter zone; and depositing and structuring a poly-Si or an α-Si layerover the opened insulation layer for use as an emitter-doping agentsource and as a contact layer; wherein, before the diffusion from theemitter-doping agent source, a doping profile is introduced into the caplayer, and the profile is low-doped on the base side with a maximumconcentration of 5×10¹⁶ cm⁻³ and highly doped on the emitter sidethereof, and wherein the low-doped region of the cap layer has athickness between 20 nm and 70 nm.
 2. The procedure of claim 1, whereinthe emitter-side high doping concentration of the cap layer does notexceed values of 5×10¹⁸ cm⁻³ when the doping agent is of the sameconductivity type as the base layer.
 3. The procedure of claim 1,wherein the cap doping profile is introduced by implantation.
 4. Theprocedure of claim 1, wherein the cap doping profile is introduced insitu during the epitaxy process.
 5. The procedure of claim 1, whereinthe cap doping profile is introduced by diffusion from the insulationlayer after highly enriching the insulation layer with the doping agent.6. A bi-polar transistor, in which structured regions consisting of acollector region and an insulation region that surrounds the collectorregion are produced on a monocrystal substrate layer, a base layer and,by means of epitaxy, a cap layer is produced over the collector zone, aninsulation layer is deposited over the cap layer, the insulation layeris opened in an area of an effective emitter zone, a poly-Si or an α-Silayer is deposited and structured over the opened insulation layer andis then used as an emitter-doping agent source and as a contact layer,wherein, in an overlapping region between an edge of the emitter windowand the outer delimitation of the structured poly-silicon or α-siliconlayer the cap layer contains a doping profile, wherein the dopingprofile comprises a low-doped region on the base side with a maximumdoping concentration of 5×10¹⁶ cm⁻³ and a higher-doped region on theemitter side, and wherein the low-doped region of the cap layer hasthickness between 20 nm and 70 nm.
 7. The bi-polar transistor of claim6, wherein the emitter-side high doping concentration of the cap layerdoes not exceed values of 5×10¹⁸ cm⁻³ when the doping agent is of thesame conductivity type as the base layer.
 8. The bi-polar transistor ofclaim 6, further comprising: a buffer layer interposed between thecollector region and the base layer.
 9. A procedure for manufacturing abi-polar transistor, comprising the steps of: producing, on amonocrystal substrate layer, structured regions consisting of acollector region and an insulation region, the insulation regionsurrounding the collector region, producing a base layer and a cap layerover the collector region, the cap layer produced by epitaxy; depositingan insulation layer over the cap layer, the insulation layer opened inan area of an effective emitter zone; and depositing and structuring apoly-Si or an α-Si layer over the opened insulation layer, and usingthis layer as a source of emitter-doping agent and as a contact layer;wherein, before diffusing from the emitting-doping agent source, adoping profile is introduced into the cap layer, the profile being lowdoped on a base side thereof with a maximum doping concentration 5×10¹⁶cm⁻³ and high doped on an emitter side thereof, and wherein thelow-doped-region of the cap layer has a thickness between 20 nm and 70nm.
 10. The procedure of claim 9, further comprising the step ofdepositing a buffer layer between the collector region and the baselayer.
 11. A bi-polar transistor, comprising: a monocrystal substratelayer; structured regions comprising a collector region and aninsulation region surrounding the collector region atop the monocrystalsubstrate layer; a base layer and, by means of epitaxy, a cap layerproduced over the collector region; an insulation layer deposited overthe cap layer, the insulation layer being opened in an area of aneffective emitter zone; and a poly-Si or an α-Si layer deposited andstructured over the opened insulation layer, this layer then used as anemitter-doping agent source and as a contact layer, wherein, in anoverlapping region between an edge of the emitter zone and an outerdelimitation of the structured poly-silicon or α-silicon layer, the caplayer contains a doping profile, and the profile is low-doped on a baseside thereof with a maximum doping concentration of 5×10¹⁶ cm⁻³ andhighly doped on an emitter side thereof, and wherein the low-dopedregion of the cap layer has a thickness between 20 nm and 70 nm.